Determination of bulk domain structure and magnetization processes in bcc ferromagnetic alloys: Analysis of magnetostriction in Fe83Ga17

Abstract

The ground state of macroscopic samples of magnetically ordered materials is a domain state because of magnetostatic energy or entropy, yet we have limited experimental means for imaging the bulk domain structure and the magnetization process directly. The common methods available reveal the domains at the surface or in electron-or x-ray transparent lamellae, not those in the bulk. The magnetization curve just reflects the vector sum of the moments of all the domains in the sample, but magnetostriction curves are more informative. They are strongly influenced by the domain structure in the unmagnetized state and its evolution during the magnetization process in an applied field. Here we report a method of determining the bulk domain structure in a cubic magnetostrictive material by combining magneto-optic Kerr microscopy with magnetostriction and magnetization measurements on single crystals as a function of applied field. We analyze the magnetostriction of Fe 83 Ga 17 crystals in terms of a domain structure that is greatly influenced by sample shape and heat treatment. Saturation magnetostriction measurements are used to determine the fraction of domains orientated along the three 100 axes in the initial state. Domain wall motion and rotation process have characteristic signatures in the magnetostriction curves, including those associated with the E effect and domain rotation through a 110 auxetic direction.

Magnetic ripple domain structure in FeGa/MgO thin films

“…However, the thin films presented here have not been treated after growth and the observed features cannot been attributed to any post-growing processing. In bulk samples, quenched in water or slowly cooled single-crystals, large domains have been observed without fine magnetic structures [19][20][21], resembling the domains obtained for films with low Ga contents seen in Figure 2a, because the magnetic contrast is only due to the presence of domain walls.…”

Transformation‐Induced Magnetoelasticity in FeGa Alloys

“…It displays a maximum, a substantial hysteresis, and a strong anisotropy. A less‐pronounced maximum was observed before and attributed to the decreasing elastic constant . The hysteresis suggests a phase transformation.…”

Non‐Joulian Magnetostriction and Non‐Joulian Magnetism

“…The most fundamental but often forgotten condition of Joule magnetostriction is zero‐volume change at all fields, regardless of the magnitude of strain (not just at saturation). Ironically, this unyielding condition of JM becomes a “diagnostic tool” to identify recent specious data claiming zero volume change in these alloys; indeed, digitization of this group's magnetostriction curves reveals a large volume change below saturation, following which, the sample inexplicably “jumps” back to strain values that somehow roughly adds close to zero.…”

“…By illustrating the most basic (but often forgotten) requirement of volume conservation in Joulian magnets, Figure b, the fallacy of some questionable recent results suggesting that NJM is not general to all Fe‐Ga alloys, is revealed. A simple digitization of curves in such literature shows a large volume change for all fields ( λ 100 + λ 010 + λ 001 ≠ 0), but this sum inexplicably changes in a near stepwise manner to zero when saturation is reached . The volume conserving condition of JM provides a simple method to identify questionable data or specious claims.…”